This invention relates to a new method of manufacturing temperature self-regulating electric circuit elements which method is fast and inexpensive and to positive temperature coefficient compositions. More particularly, the invention relates to new positive temperature coefficient ("PTC") compositions which can be annealed to attain a suitable volume resistivity in a much shorter time than those previously known.
PTC compositions and devices which exploit them are well known in the art. When a constant voltage is applied across the composition, the current and the resistance stay approximately constant so long as the PTC composition is at low temperature. When the PTC composition heats up, it reaches a "switching" temperature or temperature range where its resistance increases dramatically (six fold or more), and since the voltage is constant, current decreases. Accordingly, PTC circuits can act essentially as temperature self-regulating devices.
The problem with known heating elements comprising PTC compositions is that the costs of manufacturing are too high. Generally, thermostatic control of conventional resistive heating elements has been a more economical approach to making heating blankets and the like.
PTC compositions consist of blends of a crystalline polymer with a high conductivity material such as carbon black. See, e.g., U.S. Pat. No. 3,410,984. Early PTC compositions, such as those disclosed in U.S. Pat. No. 3,976,600, required up to 50% conductive carbon black in order to obtain the PTC effect. When such high carbon black loadings were used, a fairly sharp switching temperature could be achieved, but the resulting blend was inflexible and deteriorated in use upon thermal cycling. Lower levels of carbon black loading were tried as disclosed, for example, in U.S. Pat. No. 3,861,029 (less than 13% carbon black). One problem with this approach was that the resistivity of the carbon loaded material was too high. However, if one annealed the polymer-carbon black blend at 250.degree. F. or higher for period upwards of 24 hours it was possible to reduce volume resistivity from approximately 10.sup.9 ohm-cms to about 10.sup.3 ohm-cms.
The conductive material added to the polymer in almost all of the prior art PTC compositions is high conductivity carbon black. The only exception appears to be U.S. Pat. No. 4,277,673 which suggests using high resistivity carbon blacks rather than high conductivity carbon blacks to shorten annealing time. While other factors such as percent crystallinity of the polymer matrix have been discussed as being of importance in achieving optimum PTC results, it is believed that none of the prior art PTC compositions have achieved flexibility and sharp switching temperatures without lengthy annealing procedures.